1. Field of the Invention
This invention relates generally to tension leg platforms for offshore drilling operations, and more particularly, but not by way of limitation, to mooring systems for tension leg platforms having a plurality of tension legs comprised of threaded tubular leg elements.
2. Description of the Prior Art (Prior Art Statement)
The following statement is intended to be a prior art statement in compliance with the guidance and requirements of 37 C.F.R. SS 1.56, 1.97 and 1.98.
In deep water, e.g. five hundred feet or more, the use of bottom-founded steel or concrete structures for oil well drilling and production operations becomes quite expensive due to the high cost of fabrication and installation of such large structures. A more economical solution to the problem of providing a suitable semi-permanent site for drilling and producing operations in deep water is the use of a floating structure which is moored to fixed sea floor anchor points with vertical tension legs. Such a structure is known as a tension leg platform.
The use of pretensioned vertical mooring elements prevents vertical motion or heave of the floating structure during the passage of waves, but does permit lateral motions. The pretensioning, which is accomplished by deballasting the floating structure after the tension legs have been connected between the floating structure and fixed sea floor anchor bases, prevents the tension legs from becoming slack during passage of the troughs of waves associated with extreme environmental conditions.
The prior art discloses several systems for mooring such a tension leg platform. U.S. Pat. No. 3,648,638 to Blenkarn discloses a tension leg platform having a plurality of tubular tension legs. The tension legs of the Blenkarn apparatus are of very large diameter, from twenty to thirty inches. Those tension legs do not include a plurality of tubular tension leg elements having threaded ends. Also, the Blenkarn apparatus does not disclose anything similar to either the upper and lower connecting means or the methods of mooring of the present invention.
U.S. Pat. No. 3,976,021 to Blenkarn et al. discloses a method of installation of a tension leg platform like that of U.S. Pat. No. 3,648,638. In that method, however, the tension legs are installed in a manner very different from the methods of the present invention. In Blenkarn et al., an ocean floor anchor means is lowered from the floating platform by a plurality of guidelines. Large riser pipes, having a diameter of twenty inches or greater, are then connected between the anchor and the floating vessel and then placed under tension. Those riser pipes do not include the threaded leg elements of the present invention. Also, those riser pipes do not include anything similar to the connecting means at the upper and lower ends of the tension leg of the present invention. The methods of placing the tension legs between the floating platform and the sea floor anchor means are considerably different in the present invention as will be apparent from the following disclosure.
U.S. Pat. No. 3,355,899 to Koonce et al. discloses another method of anchoring a floating vessel. In the Koonce et al. method guidelines attached to the sea floor anchor are threaded through tubular tension legs of a combined tension leg structure which is initially located upon a barge or other floating vessel. The tension leg structure is then lowered, as a complete assembly, from the floating vessel and guided to the anchor means by the guidelines which had been threaded through the tension legs. Koonce et al. does not disclose the threaded tension legs of the present invention nor the methods of installing the same.
U.S. Pat. No. 3,618,661 to Peterman discloses a number of different guide means for connection between a drill string and a plurality of guide cables to guide the drill string toward a specific location on a sea floor anchor means.
Because these floating structures must remain in place throughout the productive life of the sub-sea oil field, and must withstand the environmental loadings expected during this time period, it is essential that the vertical mooring system components be reliable, inspectable and replaceable if need be. These requirements, first of all, demand redundancy in regard to the number of tension legs. Other desirable objectives in the selection of vertical mooring elements or tension legs are that they can be installed using onboard equipment carried on the tension leg platform, without the need of expensive derrick barges or other specialized service vessels.
These requirements are met by the methods and apparatus of the present invention which provide multiple, redundant tension legs for a tension leg platform, each tension leg consisting of individual threaded steel tubular tension leg elements which can be readily installed, retrieved for inspection, and replaced if necessary, using equipment onboard the tension leg platform.